1. Field of the Invention
The present invention relates to a liquid crystal composition, a liquid crystal device, a reflective display material and a photochromic material and in particular to a liquid crystal composition changing its selective reflection wavelength by a redox reaction, a liquid crystal device using the liquid crystal composition, a reflective display material and a light modulating material.
2. Related Art
Structural color is observed commonly in nature, such as in gloss of fish iris, peacock feather, insect shell, morpho, pearl and opal. A dye color is observed when electrons of a compound interact with visible light thereby absorbing a color of specific wavelength and reflecting other colors to be observed, while a structural color is generated when a nanoperiodic structure having a wavelength similar to or shorter than the wavelength of visible light interacts with light through interference, diffraction, refraction, scattering or the like. Accordingly, the structural color is superior to the dye in durability and safety, and in recent years, interference or the like with a pearl pigment or a multilayer film by the structural coloring principle has been used in automobiles and cosmetics.
Blue damselfish takes on bright cobalt blue by a multilayer structure of cytoplasm and a reflecting platelet and is known to instantly change the color by changing its laminate intervals upon external stimulation (that is, motility structural color). In recent years, technology for such change in biological structural color has been extensively studied (see, for example, Science, 274, 959 (1996) and Nature, 389, 829 (1997)).
Generally, structural color wavelength (selective reflection wavelength) λ is represented by the following Bragg reflection formula:Bragg reflection formula: n·λ=2d·sin θwherein n represents the refractive index of a medium, d represents the interval of a periodic structure of the refractive index, and θ is an incident angle of light.
By regulating n and d in the formula, the selective reflection wavelength λ can be regulated, and attempts at change thereof have been made by various external stimuli (light, heat, pressure, chemical stimuli, etc.).
Generally, when the structural color is applied to optical devices such as display devices including displays, electric stimuli are suitable as external stimuli, from the viewpoint of device structure and device stability.
Proposals have been made of toning of structural color by electric stimuli includes toning utilizing a change in refractive index by orientation modulation of a liquid crystal filled in inverse opal (see, for example, Physical Review, B72, 233105 (2005)), toning utilizing a change in periodic structure using a dielectric elastomer (see, for example, Advanced Materials, 17, 2463 (2005)), toning utilizing a size change of gel responding to pH change by hydrolysis of water (see, for example, Advanced Materials, 19, 2807 (2007)), and toning utilizing an opal-included polymer expanding or shrinking in response to a change in hydrophilicity and hydrophobicity by the redox reaction of ferrocenyl silane (see, for example, U.S. Serial No. 2004/0131799A1).
It is known that a cholesteric liquid crystal, similar to an opal structure and a thin-film interference structure, shows structural coloration. Its selective reflection wavelength is represented by the product of pitch length and average refractive index, as shown in the following formula 1:m×λ=P×n×cos θ  1wherein m represents a positive integer, n is the average refractive index of a liquid crystal (average refractive index: the mean of refractive indexes in long and short axes), P represents the pitch length of a helical periodic structure (pitch length: periodic distance for a liquid crystal molecule to rotate by 360° when its helical axis is observed in the axial direction), and θ represents the angle between a normal line of a sample surface and a helical axis.
In toning of structural color by a cholesteric liquid crystal, unlike an opal structure or a thin-film interference structure, a change in pitch length in the direction of liquid-crystal molecular orientation is utilized, so there is an advantage that the toning is not accompanied by volume change. Attempts at changing pitch length by external stimuli have been extensively made, and a method of changing it by heat or light has been proposed (see, for example, Chemistry Letters, 1999, 87-88 (1999) and Liquid Crystals, 27, 929-933 (2000)), while there are few proposals for the method by electric stimuli.
As the proposal by electric stimuli, a method of changing pitch length by applying an electric field or a magnetic field to a liquid crystal in a direction perpendicular to a helical axis is known (see, for example, APPLIED OPTICS, 43, 5006 (2004)).
However, this method cannot be said to be an effective method because manufacture of a device is not easy because of necessity for complicated steps in manufacture of electrodes, such as photolithography, and an extremely high voltage is required for toning because of difficult reduction in the distance between electrodes.
A method of changing apparent selective reflection wavelength by inclining the helical axis θ of a cholesteric liquid crystal, as shown in formula 1 above, has been reported (see, for example, Proc. of SPIE, Vol. 5936, 59360X-1 to 59360X-6 (2005)).
However, this method cannot be said to be a method excellent in principle because there are many problems: for example, the visual field generating selective reflection is extremely limited; the light extraction efficiency (reflectance) by interfacial reflection is significantly reduced for the reason of its principle; and application of high voltage is necessary.